Davidson's legal travails began on January 16, 2005, when she told the U.S. Air Force that her husband Michael Severance, an airman, had been missing since the day before. Air Force investigators and the San Angelo Police Department began parallel investigations, which led them to conclude it was unlikely that Severance had deserted.

Air Force Special Agent Greg McCormick did learn early on that Davidson owned a horse on a ranch, but investigators didn't know where...

it was. In hopes that Davidson would lead them to it, Air Force agents placed a tracking device on the underside of her car in the middle of the night on February 26.

One day later, "data retrieved from the device" showed that Davidson had driven to a ranch owned by Terrell Sheen, who told agents that he boarded horses including Davidson's and that they were welcome to search the property. The Air Force agents did: it proved to be an expansive ranch that included a barn, mobile homes, fenced corrals, and ponds.

Their search did not find Severance, the missing airman. On March 5, a Texas Ranger and a San Angelo sergeant interviewed Davidson at the veterinary clinic where she worked. They already knew at this point (it's unclear how they knew this) that the computer had been used to perform Internet searches on topics including polygraphs and the phrase "decomposition of a body in water." The police said Davidson became defensive when asked about the pond on Sheen's ranch and her Internet searches.

What you might expect to happen did, in fact, take place. Michael Severance's body was found in one of the ponds, with Davidson claiming she moved the body to protect a family member who might have been the murderer.

The police didn't believe her; she was charged with murder and two counts of tampering with evidence and, after a no-contest plea, was eventually sentenced to 25 years in prison. One report said Davidson owned a small animal clinic and had apparently poisoned her husband with a mix of phenobarbital and pentobarbital before weighing down his body with a boat anchor.

On appeal, Davidson's lawyers filed a motion to suppress evidence obtained from the tracking device. (It's not clear that it was a GPS-based unit, but because that's by far the most common, it's a fair assumption.) Her appeal said the tracking data should be suppressed because a judge's approval was necessary--and more specifically that it violated the Fourth Amendment, the Texas Constitution, and the Texas Code of Criminal Procedure.

Note that while her husband was a member of the U.S. military, Davidson was not, and the same Fourth Amendment standards that circumscribe the actions of local police should presumably circumscribe the actions of federal investigators as well.

If the same rules apply, what should those rules be? Courts have split on the topic; here's a review of cases I wrote in 2005. Suffice it to say that a pair of U.S. Supreme Court cases--U.S. v. Knotts and U.S. v. Karo--established that police don't need court approval to track suspects, at least when using a crude radio beeper.